This invention relates to an improved zeolite L-based catalyst and the use of this zeolite L-based catalyst in a reforming process, and more particularly to a zeolite L-based catalyst in which the catalytic activity is maintained for an increased period of time. Further, this invention provides an improved reforming process in which the improved zeolite L-based catalyst produces aromatic hydrocarbons at high yields and selectivity for an increased period of time.
The catalytic reforming reaction is conventionally used in the oil industry for converting paraffins into aromatic hydrocarbons. Conventional methods of catalytic reforming are based on the use of catalysts comprising a noble metal on a carrier. Common catalysts of this kind are based on alumina carrying platinum and optionally a second metal such as rhenium. Use of carriers other than alumina, such as X and Y zeolites have also been proposed provided the reactant and products are sufficiently small to flow through the pores of the zeolites.
In conventional catalytic reforming processes hydrocarbons, preferably C.sub.6 to C.sub.10 paraffins, are converted to aromatics by passing the hydrocarbons over the catalyst in the presence of hydrogen at operating conditions generally within the temperature range of about 400.degree. C. to 565.degree. C. and pressures varying from 200 kPa to 3.5 MPa at H.sub.2 /feed mole ratios of at least 3 and at LHSV of 0.2 to 5 W/W/HR. Part of the hydrocarbon feed is converted into aromatics by dehydrocyclization, but the reaction is also accompanied by isomerization and hydrocracking reactions. The latter converts some of the hydrocarbon feed, resulting in an undesirable loss of selectivity of C.sub.4 -gaseous hydrocarbons. The aromatic yield and selectivity of the reforming process varies with the reaction conditions and type of catalyst. High aromatic yield is desired, but not at a disproportionate loss to C.sub.4 -hydrocarbons, or at a significant lessening of catalyst life.
One particularly suitable catalyst for reforming employs a type L zeolite support which has shown to be more selective with regard to the dehydrocyclization reaction, thus providing a high yield of aromatic hydrocarbons as disclosed in U.S. Pat. No. 4,104,320. However, one problem encountered in the use of a catalyst based on zeolite L is that the catalytic activity of the catalyst cannot be maintained for an extended period of time.
During the reforming process, the zeolite L-based catalyst is deactivated fairly rapidly, for example, on the order of 50 to 150 hours, generally by the agglomeration of the noble metal on the catalyst and/or the formation of coke on the catalyst. While regeneration may be carried out, this is a time consuming and involved procedure which requires the reforming reactor to be shut down. Thus, economically a commercial reforming process typically requires the catalyst to be active at high conversion rates for extended periods of time. For a zeolite L-based catalyst it is desirable to maintain catalytic activity for at least 250 hours.
Thus, a basic feature of this invention is to provide an improved reforming catalyst employing a zeolite L support which maintains the catalytic activity (high yield of aromatics) for an extended or increased period of time.